Riverine and submarine groundwater discharges into the Mediterranean Cilician Basin and their impact on the marine water and nutrient budget

Terrestrial water fluxes and nutrient inputs are the major components of marine nutrient budget. Nutrient fluxes through surface and groundwater pathways are especially important for coastal eutrophication and pelagic productivity. Cilician Basin of the Eastern Mediterranean experiences coastal eutrophication, and the roles of surface and underground discharges have not been assessed. We conducted nutrient and ²²⁸Ra monitoring surveys during 2021 and 2022 to elucidate the relative contributions of riverine and submarine groundwater discharges (SGD) to the Cilician basin nutrient/water budget. All major rivers (8 sites) and neighboring groundwater have been monitored for nutrient concentrations and ²²⁸Ra activities monthly throughout 2022. Further, coastal groundwater wells at four different depths (0-50 m) and its neighboring river were sampled twice a week during the same time period as an intensive observation site (METU-IMS) to elucidate high-frequency temporal dynamics in the nutrient concentrations. We also conducted two basin-wide marine surveys to determine ²²⁸Ra activities in the Cilician basin water masses during dry (summer) and wet (spring) seasons. Finally, we constructed a mass-balance box model using ²²⁸Ra activities to estimate the relative flux of SGD in relation to the fluxes from regional rivers. The residence times were calculated to estimate ²²⁸Ra offshore exchange rates using a Lagrangian particle tracking model.

Nutrient monitoring around the catchment revealed very high nutrient concentrations in both river water (PO₄: 0.02-79 µM, TIN: 2.7-1302 µM, SiO₄: 5-542 µM) and groundwater (PO₄: 0.02-11 µM, TIN: 1.91-1187 µM, SiO₄: 6.6-1082 µM).  Concentrations in the river water indicate potential annual riverine N, P, Si loads of 29.1, 0.5, 19.8 Kt/year, respectively. The mean TIN:PO₄ ratio in the groundwater might be as high as c. 400, suggesting that SGD can be one of the main drivers of the Eastern Mediterranean’s phosphate limitation. The high-frequency observations in river and groundwater at METU-IMS site revealed significant variability both in N and P concentrations, which might reflect patterns in extreme weather events as well as agricultural activities. Based on a limited set of samples ²²⁸Ra activities ranged between c.40-174 dpm.m^-³ in the river water and between c. 43-257 dpm.m^-³ in the groundwater during the wet season, indicating lower activities than previous estimations.

The preliminary results of the box model simulations suggested that the total SGD can be comparable or even dramatically larger (max of 60) than the annual riverine flux into the basin. The sensitivity analyses indicated that the variability and potential overestimation of SGD flux were mostly due to the variability in marine water mass residence time estimations, measurements of groundwater ²²⁸Ra activities and the lack of saline end-member activities. We currently measure the remaining set of samples for ²²⁸Ra activities, diversify end-member samplings, and calibrate basin-wide mass-balance model to decrease the uncertainty in our estimations of the SGD budget for the region. Overall, we documented a large N and P load from Cilician Basin catchments with significant temporal intra-annual variability. Furthermore, the ²²⁸Ra activities across the Cilician basin as well as its catchments indicate the predominant role of SGD in the Eastern Mediterranean water and nutrient budget.